Mechanical structure of a humanoid robot is what holds every physical component together, serving the same duty as a human skeleton. The structure must be designed to provide mobility, stability, strength, dexterity and balance while allowing humanlike movements for the robot to function similar to a human while doing physical tasks.

The frame that serves as the skeleton can be made commonly from aluminum, carbon fiber, titanium, high strength plastic or composite materials.

Joints connect different parts of the body, serving as limbs, to facilitate movement. They must be strong enough to allow the functioning of the robot at the required rate of movement. Obtaining the desired rotations of joints is not as straightforward as it seems because in human body, limbs do not only rotate around a single axis (your fingers, elbow, knee) but they can also rotate around two (your wrist, thumb) or even three (your shoulder) axes. To achieve multiple axes movement we can combine single axis rotational actuators in series (which is by far the most common to date), use tendon or linear actuation systems that are placed precisely which should ultimately converge in design to how muscles rotate joints in human body, which is an ongoing research area. An advanced humanoid robot typically has around at least 25-30 Degrees of Freedom (DOF) – the total number of axes around which the robot can make rotations with its limbs. Degrees of Freedom is a term that actually originates from statistics. It is also commonly used in civil engineering when analyzing structures. The difference here is that in civil engineering we do not want any movement or allow it only in very limited degrees of freedom in order to release force or moment accumulation, while in the case of robots we design for moving mechanisms.

The main torso should not only be adequate to house all essential components such as on board computers, batteries, communication interfaces, but also be designed to provide stability and balance. On the main torso , arms and legs, actuators are mounted at strategic locations, in order to obtain movements as desired. Actuator mounts on the torso and arms and legs must be able to resist the transmitted forces. Overall, the center of mass of the robot should be kept as low as possible to help overall stability. For this, the heaviest component such as batteries is placed as low as possible.

The most intricate part in the frame is the hands, which is the most important to manipulate objects as effectively as a human. The hands should have enough dexterity, strength, and should be able to sense even small forces. They can be designed similar to human or be in the simpler gripper form.

Feet are often made as flat, but can also have toe joints. They typically have pressure sensors on them.

Protective cover shell is placed around the whole body for protection of internal components from environmental factors or impacts, as well as ensuring humanlike appearance and aesthetics.

By: A. Tuter

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